The invention relates to supplying a physical blowing agent into the melt flow of an injection molding machine for uniform distribution therein, characterized by high reproducibility and certainty of performance during that process so as to realize a homogenous polymer/blowing agent mixture for use in a conventional injection molding machine.
From German patent DE 24 02 203 C3 and U.S. Pat. No. 5,297,948, devices are known for the production of foamed plastic molded parts, limited to a known extrusion method wherein the blowing agent is added in only some locally defined area. This invention is a further development of the device which is claimed in our patent application EP 1 256 430 A1 for the production of foamed plastic molded parts. This said device relates to an injection molding machine which is used for the production of a foamed plastic molded part. In order to produce a foamable plastic melt, a blowing agent is added to the plastic material and generates gas bubbles in the injection molding tool as a result of an expansion of blowing agent present in solution under pressure in the melt and a pressure drop during injection into the injection molding tool, whereby the gas bubbles are frozen as the melt cools down as a consequence of an increase in viscosity and finally forming the foamed texture. The device and method which are introduced by the EP 1 256 430 A1, utilize physical blowing agents. The supply of the physical blowing agent is made possible through the use of a porous casing. This porous casing is mounted at the screw piston, preferably in the area between the metering zone and the adjacently located downstream mixing zone. The porous casing is made from a porous or permeable material which permits the expulsion of the blowing agent under pressure in order for it to dissolve within the melt. Due to its large surface, this thin-walled cylindrically shaped casing is extraordinarily suited for bringing the gas into polymer melts of different composition. The solution as introduced by the EP 1 256 430 A1 relates to a gassing step, followed by a mixing step carried out by means of a mixing element mounted at the screw piston. The gassing treatment is carried out in a sector of the screw piston, which means that the gassing elements carry out the motions of the screw piston. Due to the use of the gassing elements in only a sector of the screw piston, a conventional injection molding machine can be fitted with the suitable screw piston so that only the screw piston needs to be exchanged in the entire installation in order to carry out the production of foamed plastic molded parts, thereby greatly reducing investment costs of the entire installation. While use of gassing equipment in the screw piston is known from DE 20 53 646 B, there, the openings for the blowing agent are configured as injection nozzles terminate in the distributor head. Through the blowing agent openings, the melt is charged with the blowing agent by means of a jet stream. If the embodiment of the gassing element is a porous casing which moves together with the screw piston in axial direction and at the same time also carries out the pistons rotational motion, it can charge the melt with the blowing agent in a uniform manner, as the porous surface does not promote release of only a single jet stream; at least, a bundle of jet streams is being released, although generally, with the device according to the invention, as also in EP 1 256 430 A1, the polymer melt is supplied with blowing agent bubbles. In the immediate vicinity of the porous casing, mixing between the gas and the polymer melt remains incomplete during the gas charge since the mixing-promoting shearing forces at the outer surface of the casing, are relatively small.
However, good mixing effects can be realized by means of introducing shear forces, expansion of the melt and recirculation of the melt.
A possible means to realize this goal is illustrated in DE 101 50 329 A1. The compressed blowing agent is brought into contact with the melt by means of a static mixing element, which is mounted between the plastification aggregate and the shut-off nozzle. A porous sinter-metal surface which surrounds the mixing elements serves as a contact element between blowing agent and polymer melt. Concentration—and pressure differentials cause the uptake of the blowing agent into the melt via diffusion—and sorption processes. The homogenisation of the polymer/blowing agent mixture happens during the injection process by means of webs associated with the static mixing elements and located intermittently in the melt channel. The recirculation, distribution and expansions of the melt within the mixer promote the diffusion processes. The uptake of the blowing agent into the melt is thereby promoted with a lasting effect. A drawback of the invention as disclosed in DE 101 50 329 A1 is that the addition of the blowing agent is carried out only right in front of the shut-off nozzle. Thus, very little time remains for a complete mixing of the melt prior to being fed through the shut-off valve into the adjacent cavity. Therefore, in order to realize a complete mixing of the melt with the blowing agent, the mixing element has to be configured with a greater length; alternatively higher pressure must be employed at the mixing element, so that the blowing agent will be distributed uniformly in the polymer melt prior to reaching the cavity via the shut-off valve. Likewise, EP 1 256 430 A1 mentions as a principal drawback of static mixing elements, the shear effects thereof which can damage the polymer matrix. A further drawback when utilizing a static mixing element in the area of the screw piston is the rather complex valve control employed to control the input of the blowing agent, whereby the cost of the installation as well as its susceptibility to failure is increased. When constructing the porous casing according to EP 1 356 420A1 there exists the danger that leakages occur in the large tight surfaces during operation, so that the blowing agent will no longer be exclusively released through the porous surface of the casing but, additionally is released through leaking at the tight points into the polymer melt. In another situation, due to a malfunction in the blowing agent system, a lowering of pressure can occur, so that the polymer melt which is under higher pressure might leak, via leaking tight points, into the blowing agent supply system.